May I see you náked on webcam?
If they block you, then they are a woman. If they say yes, then they are a guy.
Isn't it pretty obvious o.O?
Jeezus - Where are you being held prisoner? ;)
Actually if you're deeply curious, one way is to hang out with them at a cafe. Order lots of beverage and see what toilet they use.
Note: This guarantee not valid in locations where restrooms are unisex. Use only as directed, and avoid looking directly into the keyhole.
sniff his crotch
Kick 'em in between the legs, with a moderate amount of force. If they weep openly, whilst crawling on the floor, then they're probably male, lol!
Maybe ask htem if they have an inney or an outey?
Say you want to see their DL picture.